In order to realize a transmission of large volumes of data in an optical transmission system, wavelength division multiplexing (WDM) has been put to practical use. The WDM can transmit data using a plurality of optical signals of different wavelengths. In other words, the WDM can transmit data using a plurality of wavelength channels.
As a technology to increase a capacity of each wavelength channel, polarization multiplexing has been put to practical use. The polarization multiplexing transmits a signal using a set of polarizations that are orthogonal to each other. In the following descriptions, the set of polarizations that are orthogonal to each other may be referred to as an X polarization and a Y polarization. Further, optical signals that are transmitted using the X polarization and the Y polarization may be respectively referred to as an X-polarization optical signal and a Y-polarization optical signal.
When a frame is transmitted in an optical transmission system, a fixed pattern region may be added to the beginning of each frame. The fixed pattern region is set by a transmitter in a frame header. A receiver can establish a frame synchronization by detecting a fixed pattern region from a received optical signal. Further, the receiver can estimate the characteristics (such as a dispersion) of an optical transmission link according to electric field information of the detected fixed pattern region.
In a polarization multiplexing optical transmission system, fixed pattern regions are inserted into an X-polarization optical signal and a Y-polarization optical signal at the same timing, as illustrated in FIG. 1. Here, a bit string of the fixed pattern region inserted into the X-polarization optical signal and a bit string of the fixed pattern region inserted into the Y-polarization optical signal are the same as each other. Thus, when the X-polarization optical signal and the Y-polarization optical signal are multiplexed, electric field information of the fixed pattern region are combined and a single polarization state is generated. In other words, a single polarization state is generated during a period of time in which a fixed pattern region is transmitted (such as 10 ns). The receiver receives the fixed pattern region in a single polarization state, so the receiver can easily detect the fixed pattern region (that is, to establish a frame synchronization) and estimate a dispersion even when an optical signal is affected by noise in an optical transmission link.
As a related technology, a method for pre-equalizing a cross-phase modulation (XPM) in an optical transmission link is proposed (for example, Japanese Laid-open Patent Publication No. 2013-228676). Further, a method for reducing non-linear optical effects that occur between channels so as to suppress a degradation in signal quality at a reception end in an optical transmission system that transmits a plurality of channels is proposed (for example, International Publication Pamphlet No. WO2015/170558). Further, in the following document, a nonlinear polarization variation due to a cross-phase modulation in a WDM transmission system is discussed. Brandon C. Collings and Luc Boivin, Nonlinear Polarization Evolution Induced by Cross-Phase Modulation and Its Impact on Transmission Systems, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, November 2000
As described above, a frame synchronization, a dispersion estimation, and the like are realized by adding a fixed pattern region to each frame. However, in a WDM transmission system, a polarization state of a certain wavelength channel affects a polarization state of another wavelength channel due to a cross-phase modulation (XPM). Specifically, when a certain wavelength channel transmits a polarization multiplexed optical signal, a fixed pattern region in a signal polarization state may cause a polarization variation in another wavelength channel. In the example illustrated in FIG. 1, a polarization state of a wavelength channel λ3 is disturbed when a fixed pattern region is transmitted in a wavelength channel λ1 during a period of time T1-T2.
During a period of time in which a fixed pattern region is transmitted in an adjacent channel (the wavelength channel λ1 in FIG. 1), a bit error may occur frequently in a disturbed channel (the wavelength channel λ3 in FIG. 1). In other words, a burst error may occur. If the bit error rate goes beyond the FEC capabilities, the receiver will not recover data correctly.